Mass analyzer including a broad range modulator



Aug. 11, 1970 R. L. WATTERS MA SS ANALYZER INCLUDING A BROAD RANGE MODULATOR Filed June 19, 1967 2 Shasta-Sheet 1 R,F. AND 0.0.

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United States Patent 3,524,150 MASS ANALYZER INCLUDING A BROAD RANGE MODULATOR Robert L. Watters, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed June 19, 1967, Ser. No. 646,947

Int. Cl. H03c 1/00 US. Cl. 332-31 Claims ABSTRACT OF THE DISCLOSURE In accordance with the present invention I provide an electronic voltage source for monopole mass filter or analyzer which is capable of rapidly sweeping through a mass range as from mass 600 to mass 1 with great stability and accuracy, and including a high voltage, high frequency voltage generator that is amplitude modulated and which includes means for providing a unidirectional component to the alternating voltage, means for accurately regulating the unidirectional voltage to a preselected fraction which conveniently may be one-tenth of the peak amplitude of the alternating current voltage, sweep means for changing the absolute values of both voltages without changing their relative magnitude, and means for regulating the voltage of said sweeping means.

The present invention relates to electrical control circuits for providing an alternating current voltage together with the unidirectional voltage that has a magnitude of predetermined proportions to the alternating current voltage which is regulated relative to the alternating current voltage and both of which are regulated to an absolute and predetermined value, and means to periodically vary said voltages in a predetermined fashion to provide a sweeping voltage. More specifically, the electrical circuits of the present invention may be utilized to provide a closely controlled and regulated voltage supply for a monopole mass analyzer or filter.

In my copending patent application Ser. No. 490,615 filed Sept. 27, 1965, now US. Pat. 3,410,998, issued Nov. 12, 1968, and assigned to the instant assignee, an electric control circuit for a monopole mass analyzer is disclosed. As pointed out therein, a monopole mass analyzer is to be distinguished from the prior analyzing devices wherein the interaction between electric and magnetic fields causes separation of the particles and the quadrupole mass analyzer in which a beam of ions is longitudinally accelerated along and within an array of four parallel rods to which balanced electrical potentials are applied. A monopole is a mass filter or analyzer in which a single conducting metallic rod is partially surrounded by a metallic shield in the form of a V which simulates the zero equipotential lines in the quadrupole. Ions are electrically accelerated axially between the metallic rod and the metallic shield. A predetermined set of voltages is applied to the conducting rod, the metallic shield normally being grounded, or at ground potential. In general the voltages applied comprise an alternating voltage with a unidirectional voltage component which component is a predetermined fraction of the peak value of the alternating current voltage as set forth in the art. Depending upon the magnitude of either or both of the applied voltages; the relative proportional relationship therebetween; or the frequency of the applied alternating current voltage, a particular mass may be chosen out of those directed along the length of the monopole for collection by a collector so that a signal is generated indicative of the concentration of the selected mass. By varying any of the aforementioned parameters different masses may be chosen to be incident upon the detector. By a predetermined and regular varying or a cyclic varying of one or more of the above parameters, a range of masses may be swept so that an analysis over a wide range of masses may be conducted in a predetermined period of time. In general, monopole analyzers or filters of the prior art have sufiered in that stability and response time has been poor and the ability to sweep a wide range of masses has been lacking. The problems attendant to the production of a stable, useful and accurate monopole analyzer are problems of producing a pair of correlated alternating and unidirectional voltages having a predetermined absolute and relative magnitude and for providing means for adjusting these absolute magnitudes as for example by sweeping, without changing the relative magnitudes so as to measure a wide range of masses. Many of these problems are overcome as disclosed in my prior application above referred to. The control circuit therein provides for analysis of a wide range of masses, for example mass 1 to mass 300, and by use of a switching device a second stage is provided wherein masses up to mass 600 may be provided. However, the control circuit therein does not provide sweep measurement of mass 1 to 600 in a single step. A primary object of this invention is to provide a simplified control circuit for a monopole mass filter or analyzer. A further object of the invention is to provide a control circuit for a mass analyzer which can measure mass 1 to mass 600 in a single step.

Another problem in my aforementioned patent application Was that to measure low mass levels, i.e., less than mass 24, it was necessary to provide a separate negative voltage supply for the control circuit. Therefore, another object of this invention is to simplify the control circuit by eliminating the need for such a separate negative voltage supply.

Another problem attendant to the use of monopole analyzers or filters has been the need for a highly regulated power supply therefor. A further object of the invention, therefore, is to provide a control circuit utilizing a more economic power supply while still providing the production of stable highly accurate voltages of unidirectional and alternating currents having predetermined magnitude relationships,

A further object of the invention is to provide an improved, highly accurate and stable source of alternating the unidirectional operating voltages for a monopole mass analyzer or filter.

Further objects and advantages of the invention will be understood from the following complete description and from the drawings wherein FIG. 1 is the diagrammatic representation of a monopole mass analyzer or filter;

FIG. 2 is a block diagram of the electrical circuit utilized to provide the controlled voltage in accord with the present invention which may be used for operating a monopole mass analyzer as illustrated in FIG. 1;

FIG. 3 is a schematic circuit diagram of the resonant circuit which produces the unidirectional voltage which is associated with the alternating current voltage; and

FIG. 4 is a schematic circuit diagram of theR-F amplifier of the voltage generator of the invention.

A monopole mass analyzer or filter is illustrated in diagrammatic form in FIG. 1 of the drawing. In FIG. 1 the monopole comprises a pair of flanged end members 1 and 2 with an evacuable cylindrical enclosure 3 connected therebetween. Enclosure 3 contains therein a I single centrally located metallic rod 4 and a V-shaped metallic shield 5 partially enclosing rod 4. A coaxial line 6 enters enclosure 3 at a central portion thereof and the center conductor thereof is connected to rod 5. A source of ions 8 is illustrated schematically by the dashed line enclosure located within the fianged end member 1 and in alignment with the space between rod 4 and shield 5. An ion detector means 7 is located at the opposite end of the enclosure 3 with flanged-end member 1 and is also axially aligned with metallic rod and shield 5. The detector means may conveniently be the electrode structure of a Dumont type 241-119 photomultiplier tube having input leads for supplying power thereto and a pair of output leads for reading information out therefrom. The ion source may be conventional and may be conveniently the same ion source as is disclosed in copending application Ser. No. 327,167 filed Dec. 3, 1963, now US. Pat. 3,230,362 issued Jan. 18, 1966, and assigned to the assignee of the present invention. The construction of the monopole tube is disclosed in detail in copending application Ser. No. 428,055 filed Jan. 26, 1965, now U.S. Pat. 3,350,559 issued Oct. 31, 1967, and assigned to the assignee of the present invention.

In the operation of the present invention ions are emitted into the space between rod 4 and shield 5 and carefully controlled interrelated alternating and unidirectional voltages are applied through coaxial line 6 to rod 4, resulting in only a single mass-to-charge ratio of ions being transmitted therethrough and incident upon detector 7 at a particular interval.

In FIG. 2 of the drawing the complete electrical circuit for the operation of a monopole analyzer or filter is shown in block diagrammatic form. The filter or analyzer is depicted generally by the ion source 8, monopole array 9, and detector 10. Correlated and carefully controlled alternating and unidirectional voltages are applied to monopole rod 4 from the resonant circuit 11. The genesis of the appropriate voltages is found in oscillator 12 which is a crystal-controlled oscillator stage for the generation of a regulated radio frequency signal. This radio frequency voltage is fed into radio frequency amplifier stage 13 which is a Class C radio frequency amplifier with control grid and screen grid amplitude modulation. The amplified radio frequency signal is fed into the resonant circuit stage 11 where a unidirectional component is taken from and added to the alternating voltage and wherein a ratio between the unidirectional voltage and the alternating voltage is closely controlled. A portion of the unidirectional voltage is taken from resonant circuit stage 11 and fed into a servostage 14, which is essentially an error circuit. Servostage 14 also receives an input from a sweep generator or other reference signal stage 15. The input supply for sweep generator 15 is a closely regulated power supply stage 17. The signal from sweep generator stage 15 to servo 14 is electrically subtracted from the sampling of the unidirectional output of the resonant circuit stage 11 and the result thereof which constitutes an error signal is fed to a servo-amplifier stage 16. After the error signal is amplified it is supplied to the radio frequency amplifier stage 13 as a modulating signal for the radio frequency alternating voltage to the resonant circuit stage 11. The actual circuits for the various stages may be of conventional design except as set forth in further detail herein.

In operation, the combination of the resonant circuit stage with the servoloop circuit 14, 16 and the sweep generator stage 15 permits automatic or manual adjustment or sweeping of the absolute magnitude of the alternating and unidirectional voltages without changing their relative proportions. This enables the monopole to pass the mass which is to be selected thereby. Thus with the exact circuits to be described herein, the mass may be swept by a factor of up to 600 to 1.

Let it be assumed that a sawtooth wave is generated by sweep generator 15 for the purpose of periodically sweeping the mass which is to be detected by detector 10. The circuit constants are so chosen that the absolute value of unidirectional voltage output that is sampled from the resonator circuit 11 and fed to the servostage 14 is essentially equal to the absolute value of the voltage delivered to the servostage 14 from the sweep generator stage 15 but of opposite polarity. As the signal from the sweep generator stage 15 changes voltage in accordance with the predetermined pattern thereof, there will be an instantaneous and very small voltage difference between these two voltages. This difference is translated by the servostage 14 into an error signal which is amplified by the servo-amplifier stage 16 and used to amplitude modulate the radio frequency signal at the radio frequency amplifier stage 13 so as to change the magnitude of the voltages generated by the resonant circuit 11 while maintaining their relative relationship. Thus a change in the voltages supplied to the monopole causes a change in the mass which is selected to be passed from the ion source 8 to the detector 10. This correction with an error signal occurs almost instantaneously and permits a rapid and constant sweeping of the mass spectrum in accordance with the voltage supplied to the servostage by the sweep generator stage.

It has been previously pointed out that it is necessary in the operation of a monopole analyzer or filter that the alternating voltage and the unidirectional voltage must be closely controlled both as to absolute and relative values to enable proper measurement of the mass. In previous systems this has generally required the close regulation of the voltage input to the radio frequency oscillator stage and to the resonant circuit stage. However, as noted in the above, the signal returned from the resonant circuit stage 11 to the servostage 14 and the signal from the sweep generator stage 15 are summed to give an error signal which adjusts porportionately the absolute magnitudes of the alternating voltage and the unidirectional voltage on the monopole in the resonant circuit stage 11. Therefore, if the absolute voltage of the signal from the sweep generator stage 15 is closely controlled the alternating voltage and the unidirectional voltage magnitudes at the monopole will be closely controlled. Such control can be achieved, for example, by energizing the sweep generator stage .15 by a regulated power supply 17. Thus, assuming any single absolute value of voltage signal input from the sweep generator stage 15, a signal from the resonant circuit 11 to the servostage 14 is summed therewith and an error signal produce-d. Irrespective of the absolute magnitude of the error signal, this signal will be amplified by servo-amplifier 16 and fed back to the radio frequency amplifier stage 13 and thence to resonant circuit 11 to adjust the absolute and relative proportions of the alternating and unidirectional voltage supplied to the monopole. Thus, instead of having to closely regulate the relatively large power supplies to the radio frequency oscillator 12 and the resonant circuit stage 11, the close controlling of the low power signal from the swep generator stage 15, as by regulating the power supply 17, is suflicient for accurate operation of the monopole analyzer or filter.

The resonant circuit stage 11 illustrated schematically in FIG. 3 of the drawing is essentially the same as the comparable circuit in my aforesaid patent, although somewhat simplified. As shown in FIG. 3, a pair of lead lines 20 and 21 which may be coaxial if needed feed an input from the radio frequency amplifier stage 13 to the resonant circuit stage, lead 21 being maintained at a high unidirectional voltage as indicated. The input to the resonant circuit is link-coupled by a step-up air transformer 22 having a primary winding 23 and a secondary winding 24. Lead 21 on primary winding 23 is maintained at radio frequency ground potential by a high voltage blocking capacitor 25. The secondary 24 of transformer 22 is adjusted to resonance with a variable capacitor 26. The resulting alternating voltage constitutes a portion of the voltage input to the monopole. The amplitude of this voltage is controlled by the magnitude of the input current from the radio frequency amplifier stage 13 and by selection of the turns ratio and coupling coefficient of transformer 22. The circuit is tuned to a frequency of 1.8 megacycles. A portion of this alternating voltage is converted to a unidirectional voltage by capacitive means. The alternating voltage sample is determined by the capacitance voltage dividing network consisting of capacitors 26, 27, the capacitance of coaxial cable 30 plus the capacitance of the monopole mass filter. The exact amount of the output voltage across capacitor 27,therefore, is predetermined so that the desired exact percentage of the voltage, for example may be sampled. This 10% fraction is then rectified by a circuit including capacitor 28 and diode rectifier 29 and a fraction of this unidirectional voltage determined by resistors 34, 35 and 36 appears on the monopole together with the alternating voltage through coaxial cable 30. Resistor 33 isolates the radio frequency alternating voltage appearing on the tank coil secondary 24 from the unidirectional voltage component selected by potentiometer 35. As in my prior application, the unidirectional voltage component is filtered of undesirable radio frequency by a series inductance 32. The 10% portion of said unidirectional voltage is applied to the servostage 14 through line 37.

The radio frequency amplifier stage 13 is illustrated in FIG. 4 of the drawing and illustrates the lead 41 from the oscillator stage 12. The oscillator stage 12 can be essentially as illustrated in my aforementioned patent application and is therefore not further illustrated. As depicted, the input to the radio frequency amplifier stage 13 is a signal having a frequency of 1.8 magacycles with an amplitude of 100 volts peak-to-peak and with a -100 volt peak. This signal is applied to control grid 42 of radio frequency amplifier tube 43, a coupling capacitor 44 being provided in line 41. The amplified error signal from the servo-amplifier stage 16 is applied to the screen grid 46 of the amplifier tube 43 through resistor 45.

The error signal is a unidirectional voltage of a magni-.

tude between 0 and +150 volts. The anode 47 of the radio frequency amplifier tube 43 is connected through output resistor 48 to the resonant circuit stage 11 through line 20. A cathode bias circuit 49 connects the cathode 50 to ground.

In order to cover the mass range 1 to 600 atomic mass units, the output amplitude of the tube 43 must similarly vary by a factor of 600 to 1. Such amplitude modulation is not ordinarily required nor attained by conventional modulator circuits which ordinarily change output amplitudes by a factor of 20 to 1. The output amplitude of the tube is further limited by the anode current which cannot exceed a maximum value without destruction of the tube. With this upper limit, it is found that the low output end of the range must be covered by driving the screen grid negative to attain measurements at the low mass ranges, for example from mass range 1 up to approximately mass 24. Unless the screen grid is negative the dynamics of the resonant circuit stage 11 is such that the absolute magnitude of the combined alternating voltage and unidirectional voltage from the resonant circuit 11 cannot be made of such a value as to determine the low mass ranges. This was accomplished by providing a separate negative screen grid voltage supply as was done in my aforementionel patent application. However, it has been found that the separate negative voltage supply is eliminated by connecting the screen grid to the oscillator supply through a resistor 51. The diode action of control grid 42 with cathode 50 rectifies a portion of the oseillator stage output to provide an average negative unidirectional voltage on the screen grid 45. Thus the voltage on the screen grid is the electrical sum of the nega tive voltage appearing across resistor 51 and the voltage from the servo-amplifier stage. Thus as the voltage from the servo-amplifier is swept from for example 0 to +150 volts the voltage on the screen grid is for example 30 to +120. Thus the amplitude of the tube covers a broad range without exceeding the upper limit of the current output for the tube. Further, for a better understanding control grid 42 to be the anode of a diode, and the screen grid 45 to be the control grid of a triode.

It is to be further noted that the interelectrode capacitance of the tube 43 must be extremely small so as to minimize direct feedthrough of radio frequency alternating voltage from control grid 42 to plate 47. Thus the tube must have the characteristic of having a control grid to plate capacitance of. 1 picofarad or less.

Thus it will be seen that there is provided a control circuit for a monopole mass filter or analyzer which provides controlled and correlated alternating and unidirectional voltages having predetermined absolute and relative magnitudes. This control circuit provides for analysis of a wide range of masses from mass 1 up to mass 600 and that such measurements may be made in a singlestep. Further, the control circuit for the above has been greatly simplified and made more economical. There has further been provided a control circuit for the above which is highly stable and well regulated from a low power regulated power supply.

While the invention has been set forth herein with respect to specific embodiments and circuit parameters it is apparent that many modifications and changes may be readily apparent to those skilled in the art. Accordingly, the appended claims are intended to cover all such modifications and changes as fall within the true spirit and scope of the invention and the specification.

What is claimed as new and desired to be secured by 'Letters Patent of the United States is:

1. A radio-frequency amplitude modulator circuit capable of a broad range of modulation comprising, means for amplifying radio frequency energy including an electric discharge device, said electric discharge device having a cathode, first and second control grids, and an anode, oscillator means applying radio frequency energy to said first control grid, means for applying a variable bias to said second control grid and means for applying at least a portion of said radio frequency energy to said second control grid whereby the modulation range is extended.

2. A circuit as set forth in claim 1 wherein said discharge device has an interelectrode capacitance of less than approximately one picofarad.

3. A circuit as set forth in claim 1 wherein said means for applying the radio frequency energy to said second control grid includes a voltage dropping resistance.

4. A circuit for producing an alternating voltage of fixed frequency having a unidirectional component thereof which bears a fixed and predetermined amplitude relation with the peak amplitude of said alternating voltage and comprising: means for generating a controlled radio frequency. signal; resonant means for producing from said radio frequency signal a high magnitude alternating voltage of the same frequency; capacitance means for taking from said alternating voltage a predetermined fraction thereof and producing therefrom a unidirectional voltage having a proportional magnitude and for adding at least a representative portion of said unidirectional voltage to said high magnitude alternating voltage; means for sampling a portion of said unidirectional voltage and comparing the same with a variable reference signal to produce an error voltage; means for amplifying said error voltage and applying said amplified error voltage to said alternating voltage signal to modulate the amplitude thereof and control the amplitude of said high magnitude alternating voltage and said unidirectional voltage without changing the proportional relationship therebetween, the improvement which comprises circuit means for closely regulating the absolute magnitude of the variable reference signal.

5. A circuit as set forth in claim 4 wherein said resonant means includes a radio frequency amplitude modulator circuit capable of a broad range of modulation comprising means for amplifying radio frequency energy including an electric discharge device, said electric discharge device having a cathode, first and second control grips, and an anode, said radio frequency generating means applying radio frequency energy to said first control grid, said means for applying the amplified error voltage being coupled to said second control grid and means for applying at least a portion of said radio frequency energy to said second control grid whereby the modulation range is extended.

6. A circuit as set forth in claim wherein said discharge device has an interelectrode capacitance of less than approximately one picofarad.

7. A circuit as set forth in claim 5 wherein said means for applying the radio frequency energy to said second control grid includes a voltage dropping resistance.

8. A control circuit adapted for energizing a monopole mass analyzer having first and second analyzing electrodes, said control circuit being adapted to produce an alternating voltage of fixed frequency having a unidirectional component thereof which bears a fixed and predetermined amplitude relation with the peak amplitude of said alternating voltage at an output thereof coupled to said analyzing electrodes comprising: means for generating a con' trolled radio frequency signal; resonant means for producing from said radio frequency signal a high magnitude alternating voltage of the same frequency; capacitive means for taking from said alternating voltage a predetermined fraction thereof and producing therefrom a unidirectional voltage having a proportional magnitude and for adding at least a representative portion of said unidirectional voltage to said high magnitude alternating voltage; means providing a closely regulated reference signal of variable absolute magnitude; means for sampling a portion of said unidirectional voltage and comparing the same with the variable reference signal to produce an error voltage; means for amplifying said error voltage and applying said amplified error voltage to said alter.- nating voltage to modulate the amplitude thereof and control the amplitude of said high magnitude alternating voltage and said unidirectional voltage without changing proportional relationship therebetween.

9. A monopole mass analyzer control circuit as set forth in claim 8 wherein said resonant means includes a radio frequency amplitude modulator circuit capable of a broad range of modulation comprising means for amplifying radio frequency energy including an electric discharge device, said electric discharge device having a cathode, first and second control grids, and an anode, said radio frequency signal generating means applying radio frequency energy to said first control grid, said means for applying the amplified error voltage being coupled to said second control grid and means for applying at least a portion of said radio frequency energy to said second control grid whereby the modulation range is extended.

10. A circuit as set forth in claim 9 wherein said means for applying the radio frequency energy to said second control grid includes a voltage dropping resistance.

References Cited FOREIGN PATENTS 357,244 1931 Great Britain.

ROY LAKE, Primary Examiner L. J. DAHL, Assistant Examiner US. Cl. X.R. 332-64 

